Same exact problem here and taken it further, Searched youtube and internet and performed and replaced every possible device and tried every possible remedy to no avail. replaced cables and contacted xfinity multiple times. They have the new App where you can chat and they try to fix it, My impression they will do anything to keep a Tech from coming out.
Conclusion, It is Xfinity not being able to handle the millions of users and if you want uninterrupted television you have to get the cable to one of their boxes.
Noticed this on channels most Viewed like last night watching the Allstar game , Buffered and buffered again and then again and etc etc.
At the time this started I was a YouTube TV subscriber and believed it had to be my again Modem/Router.
Replaced it and it was the same.
Happens on all 4 TV's with all different style roku devices.
I finally gave up and decided we will have to get cable to one television with their Box for uninterrupted Television.
I am no longer with Xfinity, but the only thing that I ever did to get rid of buffering was to restart my TV through the roku system menu.
